Preparation of alloys



TENSILE ELONGATI 0N Fig.2.

TRANS GRANUIAR FA IL URE RANGE J. b. NISBET PREPARATION OF ALLOYS Filed Aug. 26, 1949 INTERGRANULAR FA ILURE RANGE (I) soup sou/mm (NOTCLEANED) (3)501: sownou (cm/wan WITH HYDROGE AND DEOXIDIZER) (4) AuaYs or mess/v1- mvsnmou M50140 sou/rum (PART/A LLY CLEANE TYPICAL men TEMPERAkTURE ALLOY KffATURE RANGE USEFUL HIGH TEM \ RECRYSTALL/ZATIWV TEMPERATURE Inventor: James D. Nisbeo,

rssrme TEMP. /N F.

zooo'by His Attorney.

Patented Aug. 14, 1951 PREPARATION OF ALLOYS James D. Nisbet, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application August 26, 1949, Serial No. 112,558

3 Claims.

- 1 The present invention relates to the preparation of alloys and is particularly concerned with a method for obtaining'alloys characterized by exceptionally high strength, ductility and cor-.

rosion resistance at elevated temperatures.

The alloys with which the invention is concerned particularly include, those of the type commonly employed for high temperature applications and comprising a base including a plurality of metals of the group consisting of iron, chromium, nickel, cobalt, tungsten and molybdenum. In addition to the base metals, the alloy may also contain suitable alloying additions.

In the application of known materials to high temperature service. considerable trouble has resulted due to the characteristic low ductility values which lead to early intergranular rupture failures. The nucleation of grain boundary cracks resulting in such failures is believed to be primarily dependent on the properties of the grain boundary and the growth of the crack primarily dependent on the properties of the grain. of course, inseparable interdependence of these factors as well as others exist. For example, stress concentrations caused by grain boundary cracks and corrosion are also very important factors. Heretofore, practically all attention in the development of high temperature alloy materials has been directed to stiffening of the grain. While, slip rate within grains and the relative rotation of grains can be slowed down by alloying and hardening the grain thereby prolonging the rupture life of the alloy, hardening the grains to almost an absolutely rigid state does not eliminate grain boundary rupture at high temperatures. In addition, the process of grain stiffening is accompanied by severe lower temperature embrittlement.

The properties of grain boundaries and grains are affected by so-called impurities particularly oxygen and nitrogen from the atmosphere. At low temperaturu where metals fall through the grains primarily by the slip mechanism, the use of alloying ingredients to combine with the oxygen or nitrogen is relatively ineffective, whereas at high temperatures where failure takes place in the grain boundaries. impurities affect properties to a major degree because the impurities are generally rejected to this area of the lattice which is already inherently limited in its ability to accommodate flow or relative grain rotation.

A primary object of the present invention is to provide alloys free or substantially free of the impurities affecting their high temperature properties.

Another object of the invention is to provide a method of producing alloys particularly adapted for high temperature applications.

A further object of the invention is to provide a method of preparing alloys possessing good high temperature strength, good ductility at ordinary temperatures and good corrosion resistance.

Additional objects of the invention will become apparent from the following description of the invention and with reference to the accompanying drawing in which Fig. 1 illustrates one form of apparatus partially in section suitable for carrying out the process of the present invention and Fig. 2 illustrates the results obtained in accordance with the invention.

In accordance with the present invention there has been developed a process which results in entirely different and new ratios of ductility to strength at high temperatures and enables the further development of stronger high temperature alloys without completely sacrificing required room temperature ductility. The process can be applied in the manufacture of known alloys and the properties thereof improved by prolonging the time for grain boundary crack nucleation.

The process involves a combination ofsteps designed to remove the volatile impurities and reduce the oxygen and nitrogen contents of the alloy. No one of the steps comprising the process is adequate to accomplish the high degree of purity desired although each step contributes to the purifying process.

Briefly described, the first step designed to eliminate nature's atmospheric environment, vaporize volatile impurities, and reduce the gas solubility comprises melting the metal in a vacuum chamber at minimum practical pressures. A residual low pressure atmosphere of an inert gas such as helium or argon is desirable.

After the vacuum treatment, pure dry hydrogen is passed through the chamber containing the molten metal in a manner such that the sur- 3 face of the metal is contacted by the dry hydrogen or the hydrogen is bubbled through the molten metal at a pressure range which can be relatively low or above atmospheric pressure provided a suflicient flow of hydrogen is maintained to blanket the metal. The hydrogen reduces the metal oxides and the water vapor formed is pumped off with excess hydrogen. The time required is dependent on the flow of hydrogen, the metal temperature, and the oxides being reduced. This operation may be considered the intermediate cleansing process. Elements other than hydrogen can be employed for this step, provided the product of the reaction is a gas and can be pumped away. Carbon has been successfully used as the product, carbon monoxide, can be pumped off as it is formed. However when carbon is used as the intermediate cleanser," the pressure above the melt must be kept low and the temperature of the melt high to cause the reaction to go forward.

The first and second steps in the process described above cleanse the metal to a degree higher than that accomplished by either step alone and to a much higher degree than that resulting from commercial atmospheric melting practices. However, both the above steps together are not sumcient unless combined with the following step by which significant quantities of oxygen and nitrogen remaining in the melt are removed or "deactivated by final additions of very active elements which (1) combine with and slag off as oxides and nitrides and/or (2) remain within the grains to a small degree and dissolve the small quantity of oxygen and nitrogen remaining. Titanium and zirconium have been very successfully employed as final purifying additions, although other elements having a high aflinity for oxygen and nitrogen can be and have been employed. Aluminum, magnesium, vanadium, silicon, calcium, etc. can be used, however, they do not have as potent a gettering action as titanium or zirconium.

Some of the results of the process as compared with the results obtained by past practices or by one or more of the steps are schematically shown in Fig. 2. Curve I is typical of the tensile elongation in percent at various temperatures of a solid solution high temperature alloy base such as one consisting of 25 atomic percent nickel, 30% chromium, 25% cobalt and 20% iron when vacuum melted only. Note the rapid drop in elongation when, as the test temperature increases, the type of fracture changes from transgranular to intergranular. By adding step two discussed above, the ductility in the intergranular fracture temperature range can be raised to higher values as shown by curve (Ix) This type of base can be hardened, the grains stiffened, and the resulting elongation vs. temperature is similar to curve 2 at elevated temperatures.

By the three steps in the process discussed above the ductility at the temperature of intergranularr fracture for solid solution bases of the type exemplifled by curve I can be raised to very high values as illustrated by curve 3 employing tita mum or zirconium as cleansing agents. An increase in rupture life is realized by raising the level of ductility. Curve illustrates the ductility from a hardened solution made by the new technique and consisting of 35.2 per cent iron, 21.8 percent chromium, 15.4 percent cobalt, 15.4 percent nickel, 3.5 percent titanium, 0.1 percent carbon and 2.6 percent zirconium, as quenched, aged. or both.

The following tabulation gives several actual test point comparisons illustrating the results obtainable in accordance with present invention.

In the table the designation Step 1 refers to the vacuum melting step; Step 2 to vacuum melting plus hydrogen treatment and Step 3 to the combination of vacuum melting, hydrogen treatment and cleansing by addition of a deoxidizer t0 the base solution which in the case of Test 4 was four atomic percent titanium and in Test 5 was four atomic percent zirconium.

Sol. (A) refers to a solid solution consisting of atomic percent chromium and 80 atomic percent nickel; Sol. (B) to a solid solution base consisting of atomic percent chromium, 25 atomic percent nickel, 25 atomic percent cobalt, and 20 atomic percent iron; and Sol. (C) refers to a solid solution consisting of atomic percent nickel, 30 atomic percent chromium and 30 atomic percent cobalt having about the same properties as $01. (B) when treated in the same manner.

Vitallium is a commercial alloy of about 27 weight per cent chromium, 3 percent nickel, 5.5 percent molybdenum, balance cobalt except for a small amount of iron and fractional percentages of carbon, manganese and silicon.

It is to be noted that whereas Nos. 4 and 5 are saturated solutions which still can be hardened to a much higher degree, yet the strength of Nos. 4 and 5 is competitive with No. 6. The room temperature ductility of Nos. 4 and 5 is 22 to whereas No. 6 is 8%.

In the preferred form, the process of the present invention is carried out by placing the mix ture of base metals, preferably of high purity, in an induction melting furnace such as is illustrated in Fig. l as comprising a crucible surrounded by an induction coil (not shown) and placed within a vacuum chamber 2. The pressure within the chamber is reduced to less than one millimeter, for example, 200 microns, before power is supplied to the induction coil through leads 3 and 4. As the charge is heated to its melting point, the pressure within the chamber is further reduced until a final pressure of less than ten microns is obtained. During this time most of the volatile impurities, such as oxygen, nitrogen and carbon monoxide, are removed from the molten base alloy. During this and the remaining portions of the process, induction heating has the distinct advantage of stirring" the melt thereby aiding in the liberation of dissolved gas.

After a pressure of less than 10 microns has been achieved indicating that most of the readily removable gaseous impurities have been removed, a stream of pure, dry hydrogen is directed onto the surface of the melt or bubbled through the melt by means of a supply tube 5, the outlet end of which is directed into the open top of the crucible I with the tip adjacent the surface or 5 immersed in the melt. The flow of hydrogen and the rate of exhaust of the vacuum chamber are so regulated that the pressure within the chamber is less than 40 mm., preferably from 1 to 10 mm. The water vapor formed by reaction of the hydrogen and the metal oxides in the melt is carried away, from the melt as a mixture of water'vapor and hydrogen. The constant flow of hydrogen purges the chamber of substantiallyall the water vapor and any other gases so that toward the latter part of the hydrogen treatment the atmosphere within the vacuum chamber consists of hydrogen and a small amount of water vapor. At the end of this treatment, which may take from 20 to 50 minutes, a, typical alloy will contain about 0.025% oxygen, 0.0001% hydrogen and 0.01% nitrogen.

In the next step of the process while continuing the flow of hydrogen, suitable active prime setters are added for the purpose of combining with the remaining oxygen and nitrogen. Preierred prime getters include carbon, aluminum, magnesium, titanium and zirconium in amounts less than one percent by weight of the melt. Carbon will react with the oxy en to form carbon monoxide most of which will be removed from the melt at the diminished pressure. The remaining elements form oxides or nitrides which slag 01! or remain in the melt showing up within the grains of the cast product. A particularly eflective way of adding the getter is through the hydrogen inlet tube from a storage chamber (not shown) located above the vacuum chamber and connected to or forming part of the hydrogen supply line.

At the same time, any alloying additions are also made. Such additions will comprise a total of up to 20, preferably not over about 10, percent by weight of one or more metals of the group titanium, zirconium, columbium and tantalum with or without a small amount of carbon.

After at least one of the desired additions have been made, the hydrogen flow is turned oil and the pressure again reduced to less than microns for the purpose of removing as much as possible of the dissolved hydrogen. When the minimum pressure is obtained, the alloy is poured at such reduced pressure into a chill mold l previously suitably positioned within the vacuum chamber below the crucible.

In the.illustrated arrangement, the crucible I is mounted on a spindle 8 through which leads 3 and 4 pass to the induction coil. A handle or lever B mounted on the spindle 8 outside the vacuum chamber is employed to control the tilting of the crucible for pouring the melt into the mold. A sight glass It! in the top of the vacuum chamber enables the operator to observe the condition of the melt during the processing and pouring thereof.

As the metal is first heated during Step 1 of the process, it is found that the pressure will rise slightly during initial heating, indicating some evolution of gases before melting. The degree of rise is, of course, dependent on pumping speed, etc. In melting alloys of the types that contain appreciable quantities of chromium, the initial pressure rise is slight. The initial rise in pressure would be expected from several sources: Evolution of gases from the crucible, evolution of absorbed gases from the surfaces of the vacuum chamber and parts that ware heated by radiation from the heated charge, and, probably most important, the outgassing of the charge itself which takes place at low pressure.

At some temperature between room temperature and the melting point, outgassing subsides. Solubility of the gases in the metals, of course, decreases with a decrease in pressure but increases with an increase in temperature. As chromium vaporizes rapidly from the molten state at even 1000 microns, a, slight pressure rise would be expected after melting, and would probably be accentuated by the condensation of hot metal vapor on cold surfaces causing evaporation of absorbed gases. In fact each element exerts its characteristic behavior in vacuum melting, but objectionable traits, such as the high vapor pressure and aflinity for oxygen possessed by chromium, are minimized to some extent when alloyed with metals such as cobalt or nickel, which are well-behaved in a vacuum system. At the minimum pressure experienced (less than 1 micron), chromium oxidizes at hightemperatures. Hence purity cannot be obtained or maintained by lowpressure melting alone when such active elements are involved. Molybdenum, for example, oxidizes rapidly at low pressure, but the oxide is very volatile and, provided the pumping capacity of the system is adequate, the oxide is at least partially removed as it forms. It is not practical, however, to purify molybdenum by vaporizing the oxide impurities at high temperature and low pressure for the following reasons: (1) The atmosphere, unless purged with an inert gas at a partial internal pressure. is always oxidizing even at the best vacuum possible; (2) some oxygen is soluble in molybdenum in the molten state and this is a potential source of oxidation; and (3) physically trapped oxides will be retained in the melt. The heating rate should be slow enough to allow considerable diffusion of gases during heating so that equilibrium is approached at the higher temperatures. Obviously. if the metal charge were heated instantaneously to the melting point, the evolution of gases would take place essentially at the melting point.

At the melting point, difiusion is rapid and equilibrium is quickly reached. Superheating above the melting point causes an increase in solubility, and, if the melt is quenched from a very high temperature, the final gas content, although the alloy is vacuum melted, can be quite high. On the other hand if the metal is cooled slowly in the vacuum system after a superheated sojourn, evolution of gas will take place during cooling and obvious bubbling will occur upon solidification, so that the best practice seems to be to vary the metal temperature above and below the freezing point and finally rapidly superheat to the minimum temperature necessary for pouring and cast quickly.

The following tests indicate the effect of the process of the present invention on the forgeability of a given alloy. In this case an alloy was prepared by the present process by vacuum melting and hydrogen treating a base alloy of 60 atomic percent nickel, 20 percent iron and 20 percent chromium and cleansed by adding 6 percent titanium and 0.5 percent carbon. The resultant alloy was forgeable. However a second alloy of the same composition prepared in the usual manner following the best laboratory practice could not be forged due to the lack of cleanliness not obtainable when the alloying metals are merely melted together. This experiment also illustrates the fact that the present process is particularly useful in the preparation of alloys containing highly reactive elements and results in the formation of alloys having physical prop- 7 erties which difl'er markedly from alloys of the same composition prepared by the usual melting techniques.

It has previously been shownthat the process of the present invention produces alloys of improved ductility and strength at elevated temperatures. Of equal importance is the improved corrosion resistance of the resultant high purity alloys. For example, in the evaluation of auto motive and aircraft internal combustion engine valve materials, one test applied is the molten lead oxide corrosion test at 1675 F. I! the material is good in this test, as determined by the weight-loss method, then subsequent physical tests are made to evaluate valve materials. It is desirable that the weight loss after one hour exposure in the molten lead oxide be less than 5 grams per square decimeter. Frequently valves used in automobile engines and in some aircraft engine installations show higher corrosion losses than this.

Lead oxide corrosion tests on a series of ternary iron-chromium-nickel alloys prepared by the vacuum hydrogen melting technique described herein show extremely and consistently low values as is shown by the test results set forth in the following table. Particular attention is directed to the 70 and 80% iron compositions, because under ordinary melting conditions one would expect these compositions to have losses approximating 80 grams per square decimeter on this test.

Composition Loss in Heat WtJGrs/sq. dm.

Fe Cr N1 9. 90 27. 6G 62. 44 20.14 37. 52 42. 34 1. 3 19. 90 27. 8O 52. 30 2. 1 19. 67 18. 32 62.01 1. 2 19. 44 9. O5 71. 51 2 29. 30 9. 1O 61. 6O 2. 0 29.65 18.41 51.94 3.6 30. 37 37. 71 31. 92 3. 2 40. 21 28. O9 31. 7O 7. 9 39. 73 18. 5O 41. 76 4. 1 39. 27 9. 14 51. 59 7. 5 49. 33 9. 19 41. 48 6. 9 49. 92 18. 6O 31. 4S 4. 6 50. 52 28. 24 21. 24 4. 5 60. 94 28. 38 10. 68 5. 8 611. 21 18. 69 21. O9 4. 3 59. 50 9. 24 31. 27 7. 0 69. 77 9. 28 20. 95 8. 0 70. G1 18. 78 10. 6O 6. 0 80. 14 9. 33 10. 53 8. 3

By dissolving elements such as aluminum, titanium, zirconium, or magnesium the corrosion rate of the materials may be further reduced. In addition the reduction of the corrosion rate by the solution of corrosion resistant elements may be suflicient to compensate for the increase in corrosion rate normally resulting from additions of hardening elements in amounts introducing secondary phases. It is also possible to use aluminum, titanium, zirconium or magnesium as the hardening constituents. The unique properties can be obtained for materials-in the iron-chromium-nickel system as well as other metallurgical systems which inherently involve significant quantities of impurities that result from commercial atmospheric pressure melting techniques.

From the above description, it will be seen that the process of the present invention makes possible the production of high temperature alloys in which the impurities can be controlled and reduced far below the amounts inherent with existing commercial melting practices. It also makes possible the use of highly reactive elements in relatively large quantities, whereas the use of these elements has not been possible by existing commercial methods. For example, 30 to 40% chromium in a solution with iron, cobalt. and nickel can be satisfactorily melted with additions of 10 to 15% titanium or zirconium. Former commercial practices were limited to the use of chromium to not over about 25% and the use of titanium or zirconium generally not above about 1%.

What I claim as new and desire to secure by Letters Patent oi the United States is:

l. The method of preparing an alloy characterized by high strength, ductility and corrosion resistance at elevated temperatures which comprises melting together in a vacuum chamber and at a diminished pressure of less than 10 microns a plurality of metals of the group consisting of iron, chromium, nickel and cobalt, heating the melt at said diminished pressure to remove the voiatile components from said melt, reducing the oxides present in the melt by means of a stream 0! hydrogen in an amount sufficient to blanket the surface of the melt while maintaining the pressure within said chamber at a pressure less than 40 mm. by continuously removing the excess hydrogen and the water vapor formed by said reduction, introducing into the hydrogen blanketed melt a few percent of at least one of the elements of the group consisting oi titanium, zirconium and carbon, discontinuing the flow of hydrogen, again lowering the pressure within said chamber to less than 10 microns to remove from the melt substantially all of the dissolved hydrogen, varying the metal temperature above and below the freezing point thereof at such lowered pressure and thereafter rapidly heating the metal to a minimum temperature necessary !or casting and casting the melt in a chill mold at such lowered pressure.

2. The method of preparing an alloy characterized by high strength, ductility and corrosion resistance at elevated temperatures which comprises melting together by induction heating and in a vacuum chamber at a diminished pressure or less than 10 microns at least three metals oi the group consisting of iron, chromium, nickel and cobalt, heating the melt at said diminished pressure to remove the volatile components from said melt, reducing the oxides present in the melt by means of a stream hydrogen directed onto the surface of the melt in an amount sufllcient to blanket the surface 01' the melt while maintaining the pressure within said chamber at a pressure less than 40 mm. by continuously removing the excess hydrogen and the water vapor formed by said reduction, introducing into the hydrogen blanketed melt a few percent of at least one of the elements of the group consisting of titanium. zirconium and carbon, discontinuing the flow of hydrogen. again diminishing the pres sure within said chamber to less than 10 microns to remove from the melt substantially all or the dissolved hydrogen, varying the metal temperature above and below the freezing point thereof at such diminished pressure and thereafter heating the metal to a minimum temperature necessary for casting and casting the melt in a chill mold at such diminished pressure.

3. The method of preparing an alloy characterized by high strength, ductility and corrosion resistance at elevated temperatures which comprises melting together by induction heating and in a vacuum chamber at a diminished pressure of less than 10 microns at least three metals of the group consisting of iron, chromium, nickel and cobalt, heating the melt at said diminished pressure to remove the volatile components from said melt, reducing the oxides present in the melt by means 01' a stream of hydrogen directed onto the surface of the melt in an amount suillcient to blanket said surface while maintaining the stantially all of the dissolved hydrogen, varying the metal temperature above and below the freezing point thereof and thereafter rapidly heating the metal to a minimum temperature necessary for and casting the melt in a chill mold at such diminished pressure.

JALIES D. NISBET.

REFERENCES CITED The following references are of record in the ille of this patent:

UNITED STATES PA'Ili'liTN'JfS Number Name Date 428,552 Colby May 20, 1890 1,277,523 Yensen Sept. 3, 1918 1,948,316 Scott Feb. 20, 1934 2,144,200 Rohn Jan. 17, 1939 FOREIGN PATENTS Number Country Date 338,409 Great Britain Nov. 20, 1930 OTHER REFERENCES,

Transactions of theAmerican Electro-Chemical Society, vol. 32, page 179. Published in 1917 by the society, Bethlehem, Pa. 

1. THE METHOD OF PREPARING AN ALLOY CHARCATERIZED BY HIGH STRENGTH, DUCTILITY AND CORROSION RESISTANCE AT ELEVATED TEMPERATURES WHICH COMPRISES MELTING TOGETHER IN A VACUUM CHAMBER AND AT A DIMINISHED PRESSURE OF LESS THAN 10 MICRONS A PLURALITY OF METALS OF THE GROUP CONSISTIG OF IRON, CHROMIUM, NICKEL AND COBALT, HEATING THE MELT AT SAID DIMINISHED PRESSURE TO REMOVE THE VOLATILE COMPONENTS FROM SAID MELT, REDUCING THE OXIDES PRESENT IN THE MELT BY MEANS OF A STREAM OF HYDROGEN IN AN AMOUNT SUFFICIENT TO BLANKET THE SURFACE OF THE MELT WHILE MAINTAINING THE PRESSEURE WITHIN SAID CHAMBER AT A PRESSURE LESS THAN 40 MM. BY CONTINUOUSLY REMOVING THE EXCESS HYDROGEN AND THE WATER VAPOR FORMED BY SAID REDUCTION, INTRODUCING INTO THE HYDROGEN BLANKETED MELT A FEW PERENT OF AT LEAST ONE OF THE ELEMENTS OF THE GROUP CONSISTING OF TITANIUM, ZIRCONIUM AND CARBON, DISCONTINUING THE FLOW OF HYDROGEN, AGAIN LOWERING THE PRESSURE WITHIN SAID CHAMBER TO LESS THAN 10 MICRONS TO REMOVE FROM THE MELT SUBSTANTIALLY ALL OF THE DISSOLVED HYDROGEN, VARYING THE METAL TEMPERATURE ABOVE AND BELOW THE FREEZING POINT THEREOF AT SUCH LOWERED PRESSURE AND THEREAFTER RAPIDLY HEATING THE METAL TO A MINIMUM TEMPERATURE NECESSARY FOR CASTING AND CASTING THE MELT IN A CHILL MOLD AT SUCH LOWERED PRESSURE. 